The present invention relates to an interferometer strain sensor or extensometer.
One type of interferometer strain gauge includes a laser diode as the light source with a subsequently connected optical isolator and optical focusing means, an interferometer, which is alterable in its length by a strain, a single mode fibre as light guide between the optical focusing means and the interferometer, a directional coupler in the single mode fibre between the optical focusing means and the interferometer, an opto-electric converter which is connected via a multi-mode fibre to an output arm of the directional coupler, and an up-down counter arranged subsequent to the opto-electric converter for counting the electrical output signals of the opto-electric converter produced by the interference fringes as a result of the strain of the interferometer.
In a known strain sensor of this type (DE 37 15 693 C1), one of the output arms of the two-arm interferometer is connected to a graduated index lens behind which a polarisation beam splitter is arranged wherein, instead of such a combination, a fibre optic polarisation beam splitter could also be provided. The respective phase displaced sinusoidal interference signals at the outputs of the polarisation beam splitter are connected to a photo-diode, pre-amplifier combination the outputs of which are connected to an electronic read-out means by means of which the interference signals are detected and supplied, with the correct prefix sign, to an up-down counter the counting value of which indicates the elongation or extension of the fibre relative to the initial state.
For the realisation of optical switches and logic elements for applications in optical computers and signal processors, optical bistability is required. For this purpose, a varying input signal is converted into discrete output states in the form of light intensities. The basis for this is a hysteresis curve which results from the (G-u*-) input-output characteristic upon which steady values of the output intensities, which are the basis for the bistability, are obtained as a function of the input intensities or of the feedback amplification.
A bistable optical switch is known which has a two-arm interferometer that has two signal outputs of which one is connected to a photo-diode of an opto-electric converter, the output voltage of which is amplified and applied to the current supply of the light source as a control voltage. The respective interferometer arms are constructed with constant, different optical path lengths and the difference in the path lengths of the interferometer arms is so determined, and the wavelength of the input light applied to the two-arm interferometer is so alterable, that the output light of the two-arm interferometer is switched bistably between its signal outputs by the alteration of the wavelength of the input light. (DE 40 21 293 A1; U.S. Pat. No. 5,117,471; Optics Letters, 1 Dec 1991/ Vol 16, No 23, pages 1896-1898 "Bistable fiber-optic Michelsen interferometer that uses wavelength control").
It is known furthermore, to generate asymmetrical interference signals by means of a "self-mixing" interferometer. The sensor light is fed back into the laser diode where a saw-toothed intensity modulation corresponding to the movement of the reflector is brought about by a spectral line width or resonator Q-factor modulation produced by interference between an external reflector at the location of the sensor and one of the laser mirrors (Optics Letters, 1992/Vol 17, No 11, pages 1-3, "Fiber-optic Doppler velocimeter that incorporates active optical feedback from a diode laser"--authors: W. M. Wang et al.)